New WordPress Screwed Up My Site

I’m using Arthemia Theme.

I upgraded automatically to the new wp and I didn’t backup anything. I’m using the Arthemia free theme. This is the message I get when I go to my site and to my dashboard:

Fatal error: Call to undefined function: _weak_escape() in /nfs/c03/h03/mnt/56001/domains/comeuptomyroom.com/html/wp-includes/wp-db.php on line 487

Can anyone please help??? If I can go back to an old version of wordpress I would like to do that. But I’m not sure how to do that without deleting all the modifications I made to the theme and all the uploaded content.

I had a couple people suggest things.

Actually I’d try just deactivating all plugins first in the hopes that it’s just a plugin issue.

How to deactivate all plugins when not able to access the administrative menus

I tried that and the error message went away but the screen is completely blank when I go to the website or the wp-admin.

Does anyone have detailed instructions on how to restore WP to an older version??? Like a tutorial.